<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var arr = [
            [1,2,3],
            [4,5,6],
            [7,8,9]
        ]
        var arr1 = arr.reduce(
            function(rel,num){
                return rel.concat(num)
            }
        )
        var sum = arr1.reduce(
            function(a,b){
                return a+b
            },0
        )
        console.log(sum)
    </script>
</body>
</html>